Pros
Experience in Capital One's Information Based Strategy (using lots of data and number crunching to optimize business decisions) teaches skill sets that are easily transferable to a variety of industries and are in demand by other employers. Very good benefits: 401k match, 3 weeks vacation (but no one keeps track of how much vacation time you use), modern facilities. Above-average work-life balance, however it can vary from team to team. You can expect 3 to 4 60+ hour weeks each year, but the late hours are often justified and the extra effort is usually rewarded. People have a human side which makes the office an entertaining place to be.
Cons
People are high caliber (top 20%) but aren't truly elite. A previous commenter said that Capital One tends to attract people who didn't receive offers or washed out of top tier banks and consulting firms, and I have to agree with him/her. The company is big, hierarchical and slow to adapt (still uses Windows XP, MS Office 2003 in 2011). Assignments tend to be boring, formulaic, and get bogged down in bureaucracy. Middle management obsesses over meaningless analytic details. Limited space, don't expect a cubicle or desk. You will have a corner of a table in a small meeting room that may or may not have a window. There's the potential to come up with a big idea and drive it into market, but you'll spend most of your time jumping through analytic and procedural hoops that have little tangible benefit. The procedures and infrastructure act as a crutch and don't offer the same learning opportunities that may exist in less structured organizations. Promotions are frequent (30% annual promotion rate, above-average workers are promoted every 2 years) however they tend to be promotions in name only, as people continue to do the same type of work that they were doing before. Job roles have become so specialized that few people have the opportunity to see how the full value chain is integrated. Company's culture is dominated with empty buzzwords like "value chain" Can be clique-y and some people tend to hold a grudge, although this is true everywhere.